1. Field of the Invention
The present invention relates to an overvoltage protection device, and more particularly to an overvoltage protection device for a capacitor connected to an output side of a power circuit of a power IC and like components.
2. Description of the Related Art
In recent years, various types of general-purpose power integrated circuits (ICs) have been developed and used in the power supplies of electric circuits.
For example, in a power supply for energizing a brushless motor of an automobile a power IC is employed for boosting voltages. Namely, as shown in FIG. 5, an output terminal of this type of a power IC 18, is connected with a charging capacitor C, so that a DC voltage Vcc applied to the power IC 18 is boosted to a voltage Vs (Vs=Vcc+A (volts)) through a booster circuit of the power IC 18, whereby the charging capacitor C charges up to the thus boosted voltage Vs.
For example, many power IC devices used in brushless motors are adapted to 12-volt systems in which a level of the voltage Vs is defined to be Vcc+10 (volts). Consequently, the case where level of the DC voltage Vcc supplied to the power IC is 24 volts (which is the maximum level in the 12-volt systems), the thus boosted voltage Vs reaches a level of 24+10 volts (i.e.. a level of 34 volts).
The dielectric strength of the power ICs adapted for use in the 12-volt systems of power supplies depends on the widths of the in print patterns of the power ICs themselves, the dielectric strength having a maximum level of 36 volts in practical situations. Consequently, as described above, even when the power IC is subjected to a maximum DC voltage of 24 volts instead of the normal 12 volts, the power IC may not suffer an electric breakdown since the boosted voltage Vs of the power IC is still under the maximum level of the dielectric strength of the power IC.
However, in some cases, there is the necessity of having the 24-volt system's power supply energize the above-mentioned brushless motor. In such cases, the power IC suffers from the problem of breakdown, because the boosted voltage Vs reaches 46 volts (i.e., Vs=36 volts+10 volts=46 volts), which completely exceeds the maximum level of the dielectric strength of the power IC, when the maximum level of 36 volts in the 24-volt system's power supply is applied to the power IC.
In one of possible solutions to the above problem, specifications of the power IC are changed so as to be well adapted to the 24-volt system. More specifically, the widths of the print pattern of the power IC may be enlarged so as to increase the maximum level of its dielectric strength. However, this substantially increases the cost of the power IC. On the other hand, the necessity of having the 24-volt system's power supply energize the above-mentioned brushless motor is rare in contrast with the necessity of having the 12-volt system's power supply energize the brushless motor. Further, even if the above-mentioned power IC is employed in the 24-volt system without changing its specifications, the power IC is rarely subjected to a voltage exceeding the maximum level of its rated voltage, except under abnormal conditions.
In an electric circuit shown in FIG. 5, as shown in the drawing, the simplest possible way for preventing the charging voltage of the charging capacitor C from exceeding a predetermined value is the provision of a Zener diode 20a in parallel with the charging capacitor C or the provision of a Zener diode 20b at a connection between a coil L and the power IC 18.
However, in actuality, when either of the Zener diodes 20a and 20b is used, an electric current applied to the Zener diode 20a or 20b is not restricted, which results in an electric breakdown of the Zener diode 20a or 20b due to overcurrent. Consequently, used of the Zener diode 20a or 20b is not adequate.
Further, additional related arts as to the charging and discharging operation of a capacitor are disclosed in Japanese Patent Laid-Open Nos. Hei 3-105068 and Hei 5-111241. More particularly, in the former (i.e., Japanese Patent Laid-Open No. Hei 3-105068), the occurrence of unitended electric sparks in the secondary side of an ignition coil connected with the output side of a DC-DC converter is prevented by not applying any voltage to the ignition coil when a thyristor in the DC-DC converter is abnormal in operation. On the other hand, in the latter (i.e., Japanese Patent Laid-Open No. Hei 5-111241), a DC-DC converter has an output voltage which is adjustable. Consequently, these related arts do not disclose a solution to the problem inherent in the power IC as described above.